The present invention relates to communications systems. More specifically, it relates to the reliable transmission of envelope modulated data signals.
In order to transmit digital data from one device to another, as in a network or over a modem, it is generally necessary to encode digital data on a carrier signal. One approach to encoding digital data is to encode the data by modulating the amplitude of the carrier signal. In other words, the digital data is encoded in the envelope of the carrier signal.
Another approach is to encode the data by varying the frequency of the carrier signal. An example of a frequency based encoding scheme is Frequency Shift Keying (FSK), where the digital data is encoded in small changes in the frequency of the carrier signal. Yet another approach is to encode the data by varying the phase of the carrier signal. An example of phase based encoding is Phase Shift Keying (PSK), where the digital data is encoded by changing the phase of the carrier signal. Frequency and phase based encoded signals typically have a constant envelope, i.e. no information is encoded in the envelope of the encoded signal, though some envelope modulation is introduced to phase encoded signals due to the change in amplitude that occurs in shifting from one phase angle to another.
In order to obtain high data transmission rates, it is desirable to encode digital data using high density encoding schemes. The idea behind high density encoding of digital data is to encode multiple bits of information in each signal cycle of the analog carrier. Some of the highest known density encoding schemes, such as Quadrature Amplitude Modulation (QAM), utilize amplitude modulation of the carrier frequency, along with phase and frequency modulation, in order to obtain high data densities. Consequently, such high density signals do not have a constant envelope.
It is also often desirable to broadcast and receive digital data at high data rates over long distances. A common method for transmitting digital data is to impress the digital data onto an analog Radio Frequency (RF) carrier for broadcast on a radio system. Typically, a digital data signal is used to modulate the carrier frequency or frequencies in some manner to produce a RF signal containing the data.
The digital data signal used to modulate the RF carrier is typically a signal derived from encoding the binary logic values of a digital stream into either one or two signals having a number of discrete levels, each level representing a digital state (i.e. one or more data bits). One example of a digital data signal is Non-Return to Zero (NRZ).
To obtain higher data transmission rates on an RF transmission system, it is desirable to impose a high density data signal onto the RF carrier frequency. However, as noted above, high density data signals often have non-constant envelopes that encode information. The transmission of a non-constant envelope signal using a RF system presents some significant difficulties.
One difficulty with signals with information encoded in the envelope is that anything that affects the envelope of the RF transmission signal will introduce distortion to the data signal and degrade the error performance of the communication system. Consequently, non-constant envelope signals are unsuitable for use in systems that use saturating amplifiers. Also, high density signals require extremely linear amplifier performance in order to accommodate the wide range of signal amplitudes.
Furthermore, in certain cases, RF systems have a limit on the peak envelope power of the transmitter. This can be due to many causes, such as the unavailability of cost-effective linear amplification devices or regulatory constraints. Also, spectral shaping, or filtering, is often performed in an RF system prior to up-conversion and non-linear amplification in order to limit the bandwidth of the RF channel. The input signal must have a relatively constant envelope to prevent regrowth of spectral sidelobes in the RF signal during non-linear amplification.
Therefore, the need exists for a cost-effective system for transmitting high density data signals over long distances and with good error performance.
In accordance with preferred embodiments of the present invention, some of the problems associated with high density data transmission are overcome.
One aspect of the invention includes a data transmission system composed of an envelope modulating data encoder configured to receive a binary data stream at an input terminal and, responsive thereto, generate an envelope modulated data signal at an output terminal. The system also includes a mixer having a first input terminal coupled to the output terminal of the envelope modulating data encoder and a second input terminal configured to receive a mixing frequency. The mixer is configured to mix the envelope modulated data signal and the mixing frequency in order to generate upper and lower sideband transmit data signals at an output terminal of the mixer. A lowpass filter of the system has an input terminal coupled to the output terminal of the mixer and is configured to pass the lower sideband transmit data signal generated by the mixer to an output terminal of the lowpass filter. An input terminal of a frequency modulator is coupled to the output terminal of the lowpass filter and the frequency modulator is configured to frequency modulate a radio frequency carrier responsive to the lower sideband transmit data signal in order to generate a frequency modulated data signal.
Another aspect of the present invention is a data receiver system that includes a frequency modulation receiver having an input terminal configured to receive a frequency modulated data signal modulated by an envelope modulated data signal. The frequency modulation receiver is configured to demodulate the frequency modulated data signal in order to recover the envelope modulated data signal for output at an output terminal of the frequency modulation receiver. The receiver also includes a multiplier having a first input terminal coupled to the output terminal of the frequency modulation receiver and a second input terminal configured to receive a decoder data modulation frequency. The multiplier is configured to multiply the decoder data modulation frequency and the envelope modulated data signal in order to generate upper and lower receiver sideband data signals at an output terminal of the multiplier. An input terminal of a bandpass filter is coupled to the output terminal of the multiplier and the bandpass filter is configured to pass the upper receiver sideband data signal to an output terminal of the bandpass filter. An input terminal of an envelope modulated data decoder is coupled to the output terminal of the bandpass filter. The decoder is configured to demodulate data centered on the decoder data modulation frequency so that the decoder demodulates the upper receiver sideband data signal in order to obtain a binary data stream corresponding to the envelope modulated data signal for output at an output terminal of the decoder.
An embodiment of a method for transmitting an envelope modulated data signal, according to the present invention, involves mixing the envelope modulated data signal with a mixing frequency to produce upper and lower sideband transmit data signals and selecting the lower sideband transmit data signal as a transmit data modulating frequency. The method then calls for modulating an RF carrier signal with the transmit data modulating frequency in order to generate a RF data signal.
A further embodiment of the method, according to the present invention, for transmitting an envelope modulated data signal includes receiving and demodulating the frequency modulated data signal to obtain a received modulated data signal, multiplying the received modulated data signal by a decoder data modulation frequency to obtain upper and lower received sideband data signals, and selecting the upper received sideband data signal as a received modulated data signal. This further embodiment then calls for decoding the received envelope modulated data signal.
The foregoing and other features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with references to the accompanying drawings.